Which branch of the autonomic nervous system is activated during the 'fight or flight' response, leading to increased heart rate and blood pressure?

The branch of the autonomic nervous system activated during the 'fight or flight' response is the sympathetic nervous system. The autonomic nervous system is the part of the nervous system that controls involuntary bodily functions, such as heart rate, digestion, and breathing. It has two main divisions: the sympathetic and parasympathetic nervous systems. The sympathetic nervous system is specifically responsible for initiating the 'fight or flight' response, which is a rapid physiological reaction designed to prepare the body for immediate action in response to perceived threats or stress. When activated, the sympathetic nervous system releases neurotransmitters, such as norepinephrine, from its nerve endings. It also stimulates the adrenal medulla, the inner part of the adrenal gland, to release hormones like epinephrine (adrenaline) and additional norepinephrine into the bloodstream. These chemical messengers act on target organs throughout the body. To increase heart rate, they directly stimulate the heart to beat faster and with greater force. To increase blood pressure, they cause vasoconstriction, which is the narrowing of blood vessels, particularly in areas like the digestive tract, redirecting blood flow to essential organs such as the muscles, heart, and brain. This combined action of increased cardiac output and widespread vasoconstriction results in the characteristic elevated heart rate and blood pressure seen during the 'fight or flight' response.